1. Field of the Invention
This invention relates to mixture controllers for controlling mixing of air with oxygen in obtainment of breathing gas of desired oxygen enrichment and is more particularly concerned with mixture control apparatus for use with a breathing demand regulator in supplying breathing gas to an aircraft aircrew member.
2. Description of the Prior Art
When an aircraft aircrew member is exposed to low ambient atmospheric pressure at high altitude, such as is the case when an aircraft having a pressurised cabin suffers a cabin decompression above 9000 meters, to provide sufficient oxygen partial pressure to prevent hypoxia, breathing gas delivered to a face mask by way of a breathing demand regulator must comprise substantially 100% oxygen. During flight at altitudes below 9000 meters, however, it is desirable to reduce the concentration of oxygen in the breathing gas so as to provide sufficient nitrogen partial pressure to prevent atelectasis.
Breathing demand regulators are known which include a facility for entrainment of air to reduce the content of oxygen in breathing gas when the source supplying breathing gas to the regulator is one which delivers substantially 100% oxygen such as liquid oxygen system or, as is more usual in modern day aircraft, an aircraft on-board oxygen generating system (OBOGS) in which oxygen-enriched product gas is derived from a molecular sieve oxygen concentrator (MSOC). Whilst molecular sieve beds of the MSOC may be cycled to produce product gas enriched with oxygen to a concentration appropriate to maintaining oxygen partial pressure in the product gas at a constant value substantially equivalent to sea level partial pressure of oxygen in air, it is a requirement in some aircraft aircrew breathing systems that the MSOC be controlled to produce product gas of maximum concentration, usually between 90% and 95% oxygen, and that this be diluted with air in the breathing regulator to provide breathing gas of desired oxygen concentration.
U.S. Pat. No. 4,928,682 (Normalair-Garrett) is one example of a disclosure of a breathing demand regulator having a facility for entrainment of air to mix with OBOGS product gas of maximum oxygen enrichment. The source of air is aircraft cabin air which is induced to enter the regulator past a spring loaded check valve by an injector arrangement. To obtain the correct percentage mix of air with the OBOGS product gas the load at which the check valve cracks open must be maintained within very close limits. This does not present a problem when the air source is pressurised aircraft cabin air; however there now exists a requirement for filtered high pressure air bled from the compressor stage of the aircraft gas turbine engine to be used as the air source for mixing with OBOGS product gas. This presents a problem in a regulator such as is disclosed by U.S. Pat. No. 4,928,682 because satisfactory operation of the check valve cannot be obtained with the wide pressure range of engine compressor bleed air, typically 240 to 1030 kPa (35 to 150 psig).
Also, the source of air supplied to the MSOC is generally high pressure air bled from a compressor stage of a gas turbine engine, which air will also vary in pressure with demands made on the engine. As a result the pressure of product gas delivered by the MSOC will also fluctuate.